Variable gain amplifiers are commonly used in communication devices. Such amplifiers can be used, for example, in the receive path of a wireless communication device as part of an Automatic Gain Control (AGC) circuit. The variable gain amplifier can be used to maintain a range of intermediate frequency amplitudes for subsequent processing. Variable gain amplifiers may also be used in the transmit path of a wireless communication device. A variable gain amplifier may be used, for example, to set the output power of a transmitted signal.
Of course, a real amplifier does not have the characteristics of an ideal amplifier. An ideal amplifier can provide unlimited amounts of output power and can be perfectly linear. However, an amplifier typically contributes some nonlinear distortion. The amplifier may, for example, produce harmonics and higher order distortion products. Additionally, a variable gain amplifier may produce varying phase delays based on the gain value. Manufacturing differences between amplifiers and amplifiers lots may produce variations in a gain transfer function. Additionally, the amplifier gain, in dB, may not be a linear function of a control input.
For many applications, the distortion contributed by an amplifier may be negligible and may be ignored. However, for other applications, the amount of signal distortion contributed by the amplifier may adversely affect the signal quality to an extent that a communication link is substantially degraded.
For example, a transmitter in a wireless telephone system operating in accordance with the Global System for Mobile communications (GSM) standards, such as those defined in ETSI 3GPP specifications, typically operates with Gaussian Minimum Shift Keying (GMSK). GMSK is characterized as having a constant envelope. The GSM specifications have evolved to include a data mode referred to as Enhanced Datarate for GSM Evolution (EDGE). A transmitter operating in a GSM EDGE Radio Access Network (GERAN) can operate with GMSK modulation or can operate with 3π/8 Phase Shift Keying (PSK) modulation. The 3π/8 PSK modulation does not have a constant envelope, but instead, has an envelope that varies by approximately 17 dB. The variation in the amplitude envelope poses additional constraints on amplifier performance. Wireless transmitters having amplifiers that were originally configured to operate using constant envelope GMSK modulation may not perform satisfactorily with 3π/8 PSK modulation.